This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-051462, filed Feb. 26, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an antenna apparatus mainly used for a portable radio device and a radio device using the same.
A portable radio device, such as a portable telephone, a PHS terminal and a small radio base station, is often integral with an antenna (or a feed point is proximate to a housing). It is required to include an antenna in a main body of a radio device such as a plastic cover to prevent breakdown when the antenna of a portable telephone or a PHS terminal falls or to resist breakdown due to weather in case of the antenna at the radio base station.
Conventionally, an inverted F-type antenna is often used as one included in a portable radio device. FIG. 1 shows the configuration of a portable radio device employing a conventional inverted F-type antenna. An inverted F-type antenna 103 is disposed (protruded) on a metal housing 101 which includes a radio device circuit 102 consisting of a radio circuit and a signal processing circuit, and which also serves as a shield. The metal housing 101 is disposed in a plastic cover which is not shown in FIG. 1. A feed point 103a is provided at the metal housing 101. As can be seen in this example, the inverted F-type antenna 103 is low profile and small in size. It has an advantage in that good radiation characteristics can be obtained despite its proximity to the housing 101.
Normally, the performance of the built-in antenna itself tends to deteriorate since the antenna is required to be smaller in size and thinner. As shown in FIG. 1, if the inverted F-type antenna 103 is employed, the metal housing 101 is used as part of the antenna, thereby making it possible to compensate for the deterioration of the antenna performance and to, therefore, enhance it.
In this way, while the inverted F-type antenna 103 is good in antenna performance, it has a disadvantage in that it tends to receive high frequency noise leaked from the radio circuit section or the signal processing circuit of the radio device circuit 102. It is actually difficult to completely shield the radio device circuit 102 with the metal housing 101 with the configuration shown in FIG. 1 and leaked noise inevitably exists on the housing 101. Besides, because of employing the metal housing 101 as part of the antenna in case of FIG. 1, the leaked noise is directly received by the inverted F-type antenna 103, resulting in the great deterioration of communication quality. In recent years, in particular, the processing speed of the signal processing circuit increases and the difference between the radio communication frequency and the frequency of the leaked noise decreases, so that the deterioration of the communication quality due to the leaked noise from the signal processing circuit causes a grave problem.
To decrease such an influence of the leaked noise, there is proposed employing a dipole antenna 104 for a portable radio device as shown in FIG. 2. As already known, the dipole antenna does not need a ground. It is, therefore, unnecessary to directly connect the dipole antenna 104 to the metal housing 101 serving as a ground. Owing to this, even if leaked noise exists on the metal housing 101, it is possible to prevent the noise from directly flowing into the dipole antenna 104.
Nevertheless, even with the dipole antenna, a problem inevitably arises if it is practically used as an antenna built in a portable radio device. If the dipole antenna is included in the plastic cover of the portable radio device, the antenna is disposed proximate to the metal housing 101, thereby disadvantageously deteriorating antenna performance. Generally, the dipole antenna exhibits best performance when arranged in a free space where nothing is present around the antenna. Thus, if the metal housing 101 is provided near the antenna, the antenna performance deteriorates. This is because the radiation power of the antenna decreases, i.e., matching loss occurs if the dipole antenna 104 is put closer to the metal housing 101.
FIG. 3 shows the calculation results of the matching loss of the dipole antenna made by the inventors of the present invention. In FIG. 3, the horizontal axis indicates a state in which an antenna is put and the vertical axis indicates matching loss. The matching loss is one which is generated when a feed line does not match with the antenna in impedance. If so, radiation power from the antenna decreases and communication quality deteriorates. FIG. 3 shows calculation results of a case where the dipole antenna exists in a free space without a metal housing and a case where a metal housing is provided in the vicinity of the dipole antenna. If the dipole antenna exists in a free space, matching loss is as small as 0.2 dB and the antenna exhibits excellent characteristic. If the metal housing is provided in the vicinity of the dipole antenna, the matching loss increases to about 8.5 dB and the antenna characteristic clearly, greatly deteriorates.
Measures to improve the input characteristics of such a dipole antenna were already taken and an antenna known as a T-matching antenna was contrived (see xe2x80x9cAntenna Engineering Handbookxe2x80x9d, The Institute of Electronics, Information and Communication Engineers edition, pp. 114-115, 1980). FIG. 4 shows a T-matching antenna. The T-matching antenna has a structure in which a short-circuit element 113 which causes a shortcircuit between quarter wavelength elements 111 and 112 is added to a dipole antenna 110 consisting of the two quarter wavelength element 111 and 112. The short-circuit element 113 functions as an antenna impedance matching element, whereby even if the dipole antenna 110 is disposed proximate to the metal housing 101 as shown in FIG. 5, good antenna characteristics can be obtained.
The right of FIG. 3 shows matching loss if the dipole antenna which has been T-matched as stated above in the vicinity of the metal housing. Although there is a metal housing in the vicinity of the antenna, the matching loss is as small as 0.5 dB and the antenna exhibits good characteristics.
The above consideration has been given to the characteristics of the dipole antenna which has been T-matched without regard to the influence of a feed line connecting the antenna to a radio device circuit. Actually, however, it is necessary to take account of the presence of the feed line. If the feed line exists, leaked noise is transferred from the radio device circuit to the feed line and finally to the antenna, possibly damaging communication quality. To prevent this, there is proposed a ferrite core for connecting a personal computer (PC) to a display. The ferrite core has, however, relatively high capacity and is not suited to be used as the feed line of a built-in antenna.
To prevent leaked noise from being transferred from the radio device circuit to the antenna without using such a ferrite core, there is proposed arranging the dipole antenna 110 which has been T-matched in the vicinity of the metal housing 101, pulling out a feed line 114 from the surface of the metal housing 101 and putting the feed line 114 in parallel to the metal housing 101 in an electrically non-contact state as shown in FIG. 5.
Upon so constituted, if the length of the feed line 114 which has been branched from the metal housing 101 is set at a quarter wavelength, two short-circuit parallel lines are formed by the feed line 114 on the metal housing 101 and the image of the feed line 114 as shown in FIG. 5. The impedance of the two short-circuit parallel lines of a quarter wavelength viewed from a feed point is quite high. Thus, even if the current of the leaked noise flows on the feed line (particularly on the outer conductor of a coaxial feed line), the leaked noise current is cut off at a high impedance portion and not transferred to the antenna.
In this method, the length of the feed line is limited to a quarter wavelength. Normally, the degree of freedom of the length of a feed line is not necessarily ensured. In practice, the radio device circuit of a portable radio device includes not only a radio circuit section but also a signal processing circuit, an information processing circuit, a power circuit and an external interface section. In consideration of the overall optimal layout of these elements, the length of the feed line cannot be always optimized. In that respect, the above-stated method is not universal.
The inventors of the present invention calculated the magnitude of a current leaked from an antenna to a feed line when a current is fed to the feed line of the antenna to turn the antenna into a transmission state so as to evaluate the deterioration of antenna characteristic with respect to noise if the length of the part of the feed line coupling the antenna to a radio device circuit which part branched on the surface of a metal housing is changed. This is equivalent to an evaluation as to whether noise leaked from the metal housing onto the feed line flows into the antenna in a reception state.
If the current leaked from the antenna to the feed line is low during transmission, it means that the electromagnetic coupling between feed point of the antenna and each point on the feed line is weak. This indicates that even if the current of the leaked noise is spread on the feed line during reception, the noise has a smaller influence on the current at the feed point.
FIG. 6 is a graph with a horizontal axis indicating the length of a feed line and a vertical axis indicating a maximum current value on the feed line normalized by a maximum current value on the antenna. If the length of the feed line is a quarter wavelength, the maximum current on the feed line is minimum (about xe2x88x9225 dB). If the length of the feed line is not a quarter wavelength, the current of the feed line increases up to 10 dB. As is obvious from this result, the influence of the leaked noise greatly varies with the length of the feed line.
In the above method, description has been given to the influence of leaked noise on the feed line while taking the dipole antenna positioned in the vicinity of the metal housing as an example. Now, the influence on the housing will be described while taking a monopole antenna arranged on a metal housing as an example.
A helical antenna is constituted by coiling the linear element of a monopole antenna to thereby make the antenna small in size. The helical antenna is used for a PHS terminal. FIGS. 7A and 7B show the radiation patterns of a radio device model employing this helical antenna. Specifically, FIG. 7A shows the radiation pattern of the radio device model on a vertical plane (XZ plane) to the ground. FIG. 7B shows the radiation pattern thereof on a horizontal plane (XY plane) to the ground. Also, FIG. 7C shows the radio device model.
As shown in FIG. 7A, the radiation pattern on the vertical plane has peaks at xe2x88x9245xc2x0 and +30xc2x0 with respect to the horizontal direction (X direction). The radiation pattern on the horizontal plane shown in FIG. 7B is closer to an omni-directional shape but the pattern level is low. Due to this, it is not suited for establishing good communication. The reason for these radiation patterns is that the portable radio device has radiation not only from the antenna but also from a high frequency current leaked into the metal housing.
If the length of the metal housing in the direction of the Z axis is longer than a half wavelength, the phase difference between the high frequency current on the antenna and that on the metal housing is reversed and radiations in the horizontal plane cancel one another. In this way, the radiation characteristics of the antenna is changed by the radiation from the metal housing and the level in the horizontal plane becomes particularly low.
To prevent the above-stated deterioration and to improve gain in the horizontal plane, there is known a method employing a dipole antenna as already described above. There are also known other methods for increasing radiation from the antenna.
The helical antenna is formed by helically winding a linear element of about a quarter wavelength. Due to this, the actual dimension of the helical antenna in a longitudinal direction is about a tenth wavelength and the radiation quantity of the antenna is quite small. Thus, the radiation from the metal housing of the radio device is superior to that of the antenna itself, with the result that the radiation characteristics of the antenna is increasingly deteriorated by the metal housing as already stated above.
To prevent this, there is proposed a method of increasing radiation from the antenna by connecting a linear element of a half wavelength to the tip end of the helical antenna. If so, the overall length of the antenna is the sum of the length of the helical antenna and that of the half wavelength element. The antenna is, therefore, longer in practice and the radiation quantity of the antenna increases. Furthermore, since the half wavelength element is connected to the helical antenna, good matching is ensured between a feed line and the antenna as in the case of employing only a helical antenna.
FIGS. 8A and 8B show radiation patterns in a case where the half wavelength element is connected to the tip end of the helical antenna as stated above. As in the case of FIGS. 7A and 7B, FIG. 8A shows a radiation pattern on a vertical plane (XZ plane) to the ground, FIG. 8B shows a radiation pattern on a horizontal plane (XY plane) to the ground. FIG. 8C shows a radio device model. As is evident from the radiation pattern on the horizontal plan shown in FIG. 8B, the quantity of radiation increases on the horizontal plane, whereas the radiation pattern on the perpendicular plane shown in FIG. 8A has large increases at +30xc2x0 and xe2x88x9250xc2x0 with respect to the horizontal direction (X direction).
Namely, although this method can increase the quantity of radiation on the horizontal plane, the antenna is not optimum for establishing good communications since there are maximum radiations in directions other than the horizontal direction. The reason for these radiation patterns is that the helical antenna and the half wavelength element deteriorate mutual radiation. That is to say, the phase of the high frequency current on the helical antenna and that on the half wavelength element are opposite to each other and the radiation from the helical antenna and that from the half wavelength element cancel each other in the horizontal direction.
Meanwhile, in a portable radio device or the like, it is desired to make the antenna omni-directional. In that case, the influence of the feed line becomes increasingly large. FIG. 9 shows a specific example of an antenna intended to have directivity in the horizontal plane shown in FIG. 8C. The dipole antenna consists of a linear element 136 provided in proximity to the upper surface of a metal housing 131 including a radio device circuit 132 therein, a helical element 137, a half wavelength element 138 provided on the tip end of the helical element 137 and a coaxial feed line 133.
The coaxial feed line 133 connects the antenna to the radio device circuit 132. One end of an outer conductor 134 is connected to the first feed point which is one end of the linear element 136. One end of a central conductor 135 is connected to the second feed point which is one end of the helical element 137. The half wavelength element 138 is connected to the other end of the helical element 137.
In the antenna shown in FIG. 9, it is expected that the half wavelength element 138 serves as a main radiation source, whereby vertical polarized waves realizes an omni-directional radiation pattern in the horizontal plane. In that case, the helical element 137 has a smaller radiation quantity than the half wavelength element 137 and only functions as a matching circuit. The calculation results of the radiation patterns of the antenna in the ZX, ZY and YX planes are shown in FIGS. 10A, 10B and 10C, respectively. As can be seen from the results, the radiation pattern of the vertical polarized wave becomes omni-directional in the horizontal plane (XY plane) but generates ripple of not less than 2 dB.
In IMT 2000 system which is expected to be put to practical use as a communication system for portable radio devices in the near future, the antenna of a radio device as a terminal is required to be no-directional in the horizontal plane so as to realize high speed data communication and to prevent ripple of a directional pattern as much as possible. In the IMT 2000 terminal of this type, the occurrence of ripple shown in FIGS. 10A, 10B and 10C may possibly exceed an allowable limit. It is, therefore, necessary to further decrease ripple.
Now, the reason for the occurrence of the ripple will be described briefly. It is considered that ripple is generated by radiation from an unnecessary current leaked onto the surface of the outer conductor 134 of the coaxial feed line 133. As is obvious from FIGS. 10A, 10B and 10C, horizontal polarized waves, which are cross polarized waves, are generated in the respective radiation patterns on the ZX, ZY and YX planes, respectively. They are radiated from the outer conductor 134 of the coaxial feed line 133 parallel to the horizontal plane. In addition, the radiation pattern on the ZY plane shown in FIG. 10B is asynchronous about the vertical axis. This is also due to the distortion of the radiation pattern of the half wavelength element 138 by the radiation from the outer conductor 134 of the coaxial feed line 133.
As described above, in the antenna intended to be integral with the portable radio device or to be built in the radio device, it is difficult to constitute an antenna so as not to receive noise leaked from the radio device circuit. It is possible to decrease leaked noise reception quantity by employing a dipole antenna which has been T-matched and by optimizing the length of the feed line. If so, however, the length of the feed line has a smaller degree of freedom. This makes it disadvantageously difficult to ensure good communication quality while maintaining the degree of freedom of the length of the feed line and maintaining both antenna characteristics and oppression characteristics for oppressing the influence of leaked noise.
Moreover, in the dipole antenna which has one element formed into a helical element so as to make the radiation pattern in the horizontal plane omni-directional, ripple or distortion is disadvantageously generated in the radiation pattern of the antenna due to unnecessary radiation from the feed line to thereby deteriorate communication quality.
Furthermore, if the half wavelength element is attached to the tip end of the helical antenna which is a monopole antenna made smaller in size and integrated with the metal housing of a radio device, so as to improve gain in the horizontal plane, the increase of gain in the horizontal plane is disadvantageously limited due to the difference in phase between a current on the helical antenna and that on the half wavelength element.
Accordingly, it is an object of the present invention to provide an antenna apparatus and a radio device using the antenna apparatus capable of decreasing the influence of noise leaked from a radio device circuit and unnecessary radiation from a feed line and obtaining good communication quality in a configuration in which the radio device is integral with an antenna.
A related object of the present invention is to provide an antenna apparatus and a radio device using the antenna apparatus capable of increasing gain in a horizontal plane and obtaining good communication quality in a configuration in which a radio device is integral with an antenna.
To attain the above objects, an antenna apparatus according to the present invention is comprised of a dipole antenna having first and second quarter wavelength elements provided linearly in proximity to the surface of a metal housing including a radio device circuit, opposite end portions of the first and second quarter wavelength elements serving as the first and second feed points, respectively; a short-circuit element short-circuiting the first and second quarter wavelength elements; and a coaxial feed line connecting the first and second feed points to the radio device circuit. The coaxial feed line is arranged so that currents flowing into the coaxial feed line from first and second feed points through parts of the first and second quarter wavelength elements and the short-circuit element are almost opposite in phase to each other.
More specifically, the coaxial feed line has an outer conductor having one end connected to the first feed point and a central conductor having one end connected to the second feed point. The outer conductor is arranged along part of the first quarter wavelength element and the short-circuit element, and branched from the middle position of the dipole antenna of the short-circuit element in the longitudinal direction thereof. The outer conductor is electrically connected to part of the first quarter wavelength element and the short-circuit element.
In the antenna apparatus configured as stated above, currents flowing from the two feed points into the two quarter wavelength elements of the dipole antenna, respectively, are divided from the connection point with the short-circuit element to the quarter wavelength elements and the short-circuit element. The currents flowing into the short-circuit element are composed at a position at which the coaxial feed line is branched and flow into the outer conductor of the coaxial feed line as a leak current. Since the currents flowing into the short-circuit element through the two quarter wavelength elements, respectively, are almost opposite in phase to each other, a leak current on the outer conductor of the coaxial feed line becomes almost zero. Thus, the influence of the leaked noise from the radio device circuit is extremely improved and good communication quality can be realized, compared with the conventional antenna apparatus.
Another antenna apparatus according to the present invention is comprised of a dipole antenna having first and second quarter wavelength elements provided linearly in proximity to the surface of a metal housing including a radio device circuit; a short-circuit element short-circuiting the first and second quarter wavelength elements and divided into first and second segments, end portions of the first and second segments serving as the first and second feed points, respectively; and a coaxial feed line connecting the first and second feed points to the radio device circuit. The coaxial feed line is arranged so that currents flowing from the first and second feed points into the coaxial feed line through parts of the first and second quarter wavelength elements and the short-circuit element, respectively, are almost opposite in phase to each other.
More specifically, the coaxial feed line has an outer conductor having one end connected to the first feed point and a central conductor having one end connected to the second feed points. The outer conductor is arranged along the first segment of the short-circuit element and the first quarter wavelength element, and branched from the middle position between the first and second quarter wavelength elements. The outer conductor is electrically connected to the first segment of the short-circuit element and the first quarter wavelength element.
In the antenna apparatus configured as stated above, currents flowing from the two feed points into the first and second segments of the short-circuit element, respectively, are divided from the connection point with the quarter wavelength elements to the quarter wavelength elements and the short-circuit element. The currents flowing into the quarter wavelength elements are composed at a position at which the coaxial feed line is branched and flows into the outer conductor of the coaxial feed line as a leak current. Since the currents flowing into the quarter wavelength elements through the first and second segments of the short-circuit element are almost opposite in phase to each other, a leak current on the outer conductor of the coaxial feed line becomes almost zero. Thus, the influence of the leaked noise from the radio device circuit is extremely improved and good communication quality can be realized, compared with the conventional antenna apparatus.
Yet another antenna apparatus according to the present invention is comprised of a linear element provided proximate to the surface of a metal housing including a radio device circuit and having one end serving as the first feed point; a helical element having one end in the vicinity of the first feed point of the linear element serving as the second feed point; a half wavelength element connected to the other end of the helical element; and a feed line provided in parallel to the linear element in proximity to the surface of the metal housing and connecting the first and second feed points to the radio device circuit. The helical element is arranged so that at least part of the helical element overlaps the feed line.
For example, a coaxial feed line having an outer conductor having one end connected to the first feed point and a central conductor having one end connected to the second feed point is used as the feed line. The helical element is arranged to be helical above the coaxial feed line, whereby a current almost opposite in phase to that flowing through the surface of the outer conductor flows through the helical element. That is, the helical element is arranged to form a radiation field canceling unnecessary radiation resulting from the current flowing through the surface of the outer conductor.
In this antenna apparatus, the helical element is put proximate to the feed line in almost parallel to each other in the vicinity of the feed points. The unnecessary radiation caused by the current on the feed line is canceled by the current flowing through the helical element, whereby the ripple of the radiation pattern in the horizontal plane is reduced and good non-directivity in the horizontal plane can be obtained.
Still another antenna apparatus according to the present invention is comprised of a linear antenna having a length of a half wavelength and provided in the vicinity of a metal housing including a radio device circuit; a quarter wavelength element having one end connected to the proximal end of the linear antenna; and a feed line connecting the other end of the quarter wavelength element to the radio device circuit. One end of the quarter wavelength element is connected to the proximal end of the linear antenna at a position below the upper end of the metal housing and the other end of the quarter wavelength element is connected to the feed line on the upper end of the metal housing.
In this antenna apparatus, the current on the linear antenna having a length of a half wavelength is opposite in phase to that on the quarter wavelength element having a length of a quarter wavelength. If viewed from the other end of the quarter wavelength element which is a feed point, the linear antenna is directed upward and the quarter wavelength element is directed downward. The electromagnetic field radiated from the linear antenna and that from the quarter wavelength element have, therefore, the same phase, and the radiation level, i.e., gain on the horizontal plane increases.
Furthermore, the present invention provides a radio device having the antenna apparatus, the radio device circuit and the metal housing stated above.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.